Well bores require periodic maintenance to remove for example accumulated sediments or for a host of other reasons well known in the industry. When maintenance is required, it is the usual practice to remove existing pumping equipment from the wellhead, and to move in a service rig to maintain control over the well during servicing and to inject and remove the necessary tools and equipment required to complete the maintenance or servicing operations.
For well servicing and workovers, the use of coiled tubing is preferred. Coiled tubing is a single length of continuous unjointed tubing spooled onto a reel for storage in sufficient quantity to exceed the maximum depth of the well being serviced. Coiled tubing is favoured because its injection and withdrawal from the well can be accomplished more rapidly compared to conventional jointed pipe, and it is particularly well suited for use in underbalanced wells. However, as with conventional pipe, service fluids and wire lines for downhole tools and instruments pass through the tubing's interior. The tubing is wound on a reel or spool mounted on a wheeled trailer or the flatbed of a truck for transport. The coiled tubing unit will normally also include an injector for insertion and removal of the tubing from the wellbore and a guidearch which leads the tubing into the injector.
For a typical cleanout, the tubing is injected into the well and a pressurized fluid is pumped through the tubing to circulate the well contents out through the annulus between the tubing and the well bore. The fluid can be a liquid but is often an inert gas such as air, nitrogen or carbon dioxide.
As a cleanout fluid, air has the obvious advantage that it costs nothing and it works reasonably well particularly in shallow wells of less than 1200 meters in depth. In shallow wells, the ratio of oxygen to hydrocarbons is not critical and there is relatively little risk of explosion. In deeper wells however, partial pressures increase, and the concentration of oxygen and its reactiveness increase sharply. This creates a real risk of explosion and the oxygen's reactiveness can cause sever corrosion by the oxidation of metallic surfaces.
Another disadvantage to the use of air is that the equipment needed to compress and pump it adds substantially to the weight of a coiled tubing rig. A major issue with coiled tubing units is the amount of coil they can carry without exceeding load limits on both the trailer and public roadways. So called “bob tailed” coiled tubing units incorporate the air compressor. The compressors typically pump 300 to 650 standard cubic feet per minute (scfm) at a maximum pressure of approximately 2000 psi. This is not sufficient in itself to blow sand from deeper wells. To add more lifting capacity, soap is added to the air stream which produces a foam. The soap is stored in a tank, and the tank and compressor combined weight approximately 7500 lbs. (approximately 3400 kg), which reduces the amount of coil the unit can carry by the same amount. This limits deeper well applications.
These and other factors mitigate against the use of air for deep well applications and favour the use of nitrogen. Nitrogen is inert at all depths and creates a safer working environment around hydrocarbons. Its also non-corrosive. It is pumped at a volume of up to 1500 scfm at pressures up 5000 psi which is sufficient to blow sediments from the wellbore without the need for soap.
To complete a job using nitrogen, both a coiled tubing unit and a nitrogen unit are required on location. The two units are rigged together at the well site and as the coiled tubing is run into the well, the nitrogen is pumped through the tubing to extrude any fluids and/or solids accumulated in the well.
Nitrogen is normally stored and transported to the site as a liquid in a pressurized container forming part of the nitrogen rig, which also includes a tractor for moving the rig from job to job, a pumping unit and a heating unit to vaporize the nitrogen prior to injection through the coiled tubing and into the wellbore. The heater is normally an open flame unit and by regulation it must therefore be kept at a predetermined safe distance from the wellhead.
The above described setup has numerous disadvantages. Most obviously, operating costs for two rigs are high because of the extra personnel, fuel and equipment required. There is the added pollution and cost resulting from the use of two tractor units and an open flame heater. The mandated separation of the nitrogen and coiled tubing units greatly enlarges the footprint at the well site which sometimes necessitates enlarging the site. The high pressure tubing delivering the nitrogen gas to the coiled tubing unit is a hazard and setup and breakdown times before and after the job are increased.